The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In the construction and development of wells formed in subterranean formations, such as wells for the production of oil and gas, various operations are carried out that require the introduction of fluids of different types into the wellbore and/or into formation surrounding the wellbore.
In acid-fracturing jobs, for instance, it is desirable to pump a gelled fluid to create the fracture initially for the acid treatment. Compared to a simple non-gelled acid treatment, the gelled fluid facilitates the creation of wider and longer fractures for the acid to etch. Additionally, by gelling the acid, this also limits acid leak-off into the formation, thus providing more effective use of the acid. The gel also reduces the acid contact with the formation by limiting worm-hole development, thereby slowing the acid reaction so that the acid has a better chance to penetrate into the fracture before reacting. This facilitates the transport of live acid to the fracture tip, increasing the effectiveness of the stimulation.
Some of the current practices of acid fracturing involve pumping a pad fluid formed from a hydratable polymer that is crosslinked to form a gel, which is then followed by the acid treatment. The treatment may involve alternating between pumping the polymer gel and acid to achieve the desired etched fracture, and resulting fracture conductivity. Such operations utilizing alternating stages of polymer gel and acid may be complicated and damage to the formation by the polymer gel.
In certain applications, gelled acid systems that are gelled with polymeric materials have been used. In polymer gelled acid systems, the viscosities are usually not very high, and temperature stability can be a problem. Furthermore, the polymer of the gelled acid system is difficult to remove, and can still damage the formation.
Viscoelastic surfactants (VES) have been used in fluids for many well treatment operations. Viscoelastic surfactants can be used to provide a high viscosity or gelled fluid and can be readily removed from the formation so that they do not damage the formation the way polymer systems can. Not to be limited by theory, but viscoelastic surfactant systems under proper conditions form long rod-like or worm-like micelles in aqueous solution. Entanglement of these micelle structures provides viscosity and elasticity to the fluid. When these fluids are subjected to shear, the shear disrupts the micelle network so that the fluids exhibit shear-thinning behavior. When the shear rate is eliminated, or reduced, the apparent viscosity of the fluid recovers as the VES micelle network reassembles.
Viscoelastic surfactant acid systems have also been developed. In particular, viscoelastic diverting acids (VDA's) have been used in many formation treatments. VDA's do not have much viscosity before the acid is spent, however, and thus may not be used as a gel system for fracturing. Alternatively, a polymer pad may be used in some VDA treatments. The use of a polymer pad may defeat the benefit of the VDA, however, because it is no longer a polymer-free system.
Accordingly, a gelled acid system that utilizes viscoelastic surfactants would be of great benefit in the treating of subterranean formations in oil and gas production, particularly for use in acid fracturing.